The present invention pertains to bi-directional driver/receiver (transceiver) circuits.
In any system employing integrated circuits (chips), information must be transferred from one chip to another. This is true whether the chips reside on a single module or on different modules. Such information transfer over uni-directional paths is inefficient because it utilizes too many pins on each chip. In particular, two-way information exchange between two chips using uni-directional transfer paths requires two separate pins on each chip: one pin on each chip for the outgoing information and a separate pin on each chip for the incoming information.
One method used in the art for reducing the number of pins utilized on each chip for chip-to-chip communications involves using a single path for transferring information in both directions, i.e. bi-directional information transfer. However, in this method, information is transferred in only one direction at any one time. Although this method reduces the required number of pins per chip to one, the transfer circuits must include specialized circuitry which monitors the chip-to-chip communication and, when necessary, switches the direction of information flow between the chips.
As a result of the above, a system's performance would be improved by using a driver/receiver (transceiver) circuit that simultaneously transmits and receives information while using a single pin on each chip. Such a bi-directional transceiver would advantageously reduce the number of pin connections required for bi-directional information interchange between chips, thereby reducing the number of chip overflows on a module, and would reduce the time required to transfer information in both directions.
An article entitled "Simultaneous Bidirectional Transceiver Circuit" by A. Y. Chang and P. J. Pandya, in IBM Technical Disclosure Bulletin, Vol. 23, No. 4 September 1980, pp. 1435-1437 discloses a bi-directional transceiver which is capable of simultaneously transmitting and receiving digital information over a single cable without using time division or frequency multiplexing. Further, the transceiver can be used to transmit and receive information between chips on the same or different modules on the same board or on different boards.
Specifically, the disclosed transceiver consists of two circuits disposed on opposite ends of an interchip cable. The data to be transmitted across the interchip cable are simultaneously applied to the circuit on the appropriate side of the interchip cable. The data applied to the first circuit is received at the output of the second circuit at the same time that the data applied to second circuit is received at the output of the first circuit.
Each of the circuits disposed at the ends of the interchip cable contains a current switching circuit whose output depends on two input voltages. For each circuit, the first input voltage is derived, by means of a first d.c. bias network with a split resistor, from a trilevel voltage generated at the end of the interchip cable and the second input voltage is derived, by means of a second d.c. bias network with a split resistor, from the data input signal. Because of the manner in which these input voltages are derived, the disclosed transceiver: (1) draws a ground current which can generate noise and disturb other nearby receivers, (2) dissipates an undesirably high power, and (3) is sensitive to switching noise, i.e. noise generated when the data switches between its states, for example between a logical "1" to logical "0" or vice versa.
As a result, a need exists in the art for a bi-directional transceiver circuit that requires no ground current, uses less power than previously disclosed circuits, and generates little switching noise.